Thunderbird on "[Plugin: WP SlimStat] Database error - autopurge_interval"

Hi,

I use wp-slimstat for my wordpress website. Only the last week i got some error(s) about the wp-slimstat. Can you take a look about this code?

Can you tell me also what this means and can this be fixed?

[11-Aug-2013 10:40:29] WordPress database error Unknown column '$autopurge_interval' in 'where clause' by query DELETE ts FROM wp_slim_stats ts WHERE ts.dt < UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL $autopurge_interval DAY)) made by do_action_ref_array, call_user_func_array, wp_slimstat::wp_slimstat_purge

http://wordpress.org/plugins/wp-slimstat/